LinkedIn has done a masterful job of establishing itself as the professional network — the top place to post résumés and connect with business colleagues. The network has done an even better job of monetizing its 175 million members’ résumés into a $1 billion-dollar business with $10 billion in market capitalization.

There is just one problem: Facebook is catching up.

As CEO of Reachable, a social business application that leverages information from both networks, I have had a front-row seat for the LinkedIn vs. Facebook battle in the professional contacts arena. Reachable builds out the professional network for an entire company including all its employees, and with permission, accesses data from numerous sources, including email systems, CRM systems, and social networks. As a result, we can track where our customers store their professional contacts.

Over the last year, I have noticed an interesting trend: Facebook is gaining as a source of professional connections. Last year, about 95 percent of new Reachable customers uploaded their LinkedIn connections, indicating that LinkedIn was their default professional network and the primary source of their professional contacts. In March of this year, only about 48 percent of our users uploaded Facebook friends, meaning they considered the network a less valuable source of professional connections.

However, over the last nine months, I have seen Facebook steadily increase as a source of professional connections: Now, 65 percent of Reachable users upload Facebook friends as professional contacts, a 35 percent increase over that period. Total LinkedIn uploads have dropped to 88 percent. The percentage is still very high, but the trend is clear: Facebook is catching up as a source of professional connections.

I think this is an inevitable trend. Facebook has become the center of many people’s online existence and their primary communication platform; user stats show they spend hours each day on Facebook. LinkedIn is a place where people post their résumés and make a few business connections, but they only spend minutes per month there. This difference in time invested on the sites is why Facebook will ultimately displace LinkedIn as the default professional network.

We have heard from many of our customers that LinkedIn has several shortcomings as a source of connections. Due to the limited time users typically spend on the site, their LinkedIn networks can’t possibly represent everyone they know professionally. Most people don’t aggressively connect with co-workers and associates unless they are actively looking for a job. And those who do aggressively connect with others and accept every connection request wind up with a network of people they don’t really know — which does not help them when they attempt to use it.

We call this the “Curse of the Casual Connection.” By connecting to someone they don’t really know or accepting a request from a stranger, LinkedIn users may feel like they are “building a professional network.” But all they are really doing is polluting the network of people they really know with strangers. These casual connections rarely develop into a significant relationship because users spend so little time on LinkedIn.

Most users track and curate their Facebook friends much more carefully because they spend more time on Facebook, are more engaged with their friends and share more information there. Facebook users are less likely to share photos and life’s moments with complete strangers, which is why the bar for friending is typically higher on Facebook. With LinkedIn, there is little risk to connecting with a stranger since there really isn’t much interaction or sharing taking place. LinkedIn users don’t curate their connections as carefully because they rarely visit the site. The bar, and hence the quality, of connections is lower.

Another factor is that Facebook users have finally realized that their Facebook friends are professionals too, with jobs and connections that can help them find a new job or introduce them to business opportunities.

Looking ahead, Facebook will continue its rise as a legitimate source of professional contacts. It is the center of many people’s online lives and they will continue to spend a tremendous amount of time on Facebook. LinkedIn will continue to serve as a place where users publicly post résumés when looking for a job, but due to limited user engagement and interaction on LinkedIn, Facebook will ultimately surpass it as the go-to site for professional networking and contacts.

Based in Palo Alto, Al is the CEO of Reachable, Inc. Prior to joining Reachable in 2011, he was CMO and SVP at Taleo. Before that, he was the founder and CEO of Jaspersoft. You can follow @alcampa on Twitter here.

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